1. Field of the Invention
The invention relates to the field of discharging capacitors, particularly where the charge remaining on the capacitor could have safety implications.
2. Prior Art
Often in homes and businesses, the power line voltage contains numerous high-frequency components not associated with, for instance, the 60 Hz alternating current (AC power) or its harmonics. These high-frequency components are caused, for example, by switched power supplies, dimmers, motors, and other sources. There are numerous good reasons why these high-frequency components are undesirable and should preferably be removed from the power line.
One way of reducing the high-frequency components is to connect a capacitor across the power line to shunt out the high-frequency components. An ordinary plug may be used to connect the capacitor to the power line at a power receptacle. This is convenient since it does not require the permanent or hard-wiring of the capacitor into the power circuit. When the capacitor is unplugged, a charge will typically remain on the capacitor and can cause a shock if, for example, a hand comes in contact with the prongs of the plug.
As will be seen, the present invention provides a circuit for discharging the capacitor once it is unplugged from the power line.
A circuit similar in structure, but not function, to the circuit of FIG. 1 is sometimes used in AC dimmers. In the dimmer application, the AC signal is phase shifted through a variable resistor and used to trigger a triac. As will be seen with the present invention, the circuit of FIG. 1 is triggered only with DC signals and is used to discharge a capacitor, not to control an AC signal for a light, or the like.
A method and apparatus for discharging a capacitor when the capacitor is unplugged from an AC receptacle is described. An attenuation circuit provides attenuation for the AC power signal and substantially less attenuation to a DC signal. When connected to the AC power line, the output of this circuit is low enough to not cause triggering of a triac since the output remains lower than a predetermined threshold voltage. However, when the capacitor is disconnected, the full DC potential remaining on the capacitor is coupled to a triggering mechanism for a triac. A discharge circuit, which includes the triac, causes the capacitor to be discharged when the DC signal is greater than the predetermined voltage.